drm/amd/display: Compare v_front_porch when checking if streams are synchronizable

[Why]
If the front porch of the two timings differ, then there may not be
enough time while both streams are in vertical blank to perform a memory
clock change. This can hang the system.

[How]
Check the each streams timing.v_front_porch when determining if the two
streams are synchronizable.
Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -399,6 +399,10 @@ bool resource_are_streams_timing_synchronizable(
 				!= stream2->timing.v_addressable)
 		return false;
 
+	if (stream1->timing.v_front_porch
+				!= stream2->timing.v_front_porch)
+		return false;
+
 	if (stream1->timing.pix_clk_100hz
 				!= stream2->timing.pix_clk_100hz)
 		return false;